In bone, dentin and other tissues, collagen type I or similar proteins assemble into a fibrillar matrix, which in some instances serves as a scaffold for the incorporation of mineral crystals. The adjacent cells establish specific contacts to the matrix, which are mediated by interactions between domains in extracellular proteins such as collagen and receptors of the cell surface, for instance integrins. Peptide domains which are involved in these contacts have been identified in several extracellular proteins (Yamada & Kleinman, 1992). In enamel, a structural network which is comparable to the collagen fibres of bone, cartilage and dentin has not been found. Also, no sequence segments have been identified in the enamel matrix proteins, which could mediate its anchoring to cell adhesion molecules. The enamel proteins amelogenin and enamelin do not contain such protein domains. The mineral content of newly deposited enamel is around 15% of the total mass and increases later, under degradation of the proteins, to 95% (Robinson et al., 1988).
Two predominant groups of proteins have been identified in enamel: enamelins and amelogenins (Termine et al., 1980). Protein fragments in mature enamel are similar to one of the enamelins, tuftelin, which has been located by antibodies in-between the enamel prisms. The cDNA sequence corresponding to tuftelin has been determined, and it has been speculated that this protein might have a function in the mineralization of enamel (Deutsch et al., 1991). The significance of the remaining, so far described, enamelins for enamel formation may be disputed, because the main protein species are identical to proteins from the bloodstream (Strawich & Glimcher, 1990). It is still discussed whether amelogenin, the most frequent enamel protein, provides a scaffold for the enamel matrix (Simmer et al., 1994).
Partial sequences of randomly selected cDNA clones from a rat in situ library have previously been compiled (Matsuki et al., 1995), of which some show homology to sequences of the invention. No reading frame was suggested from the partial sequences. It was not stated if polypeptides are encoded by these sequences and no suggestion as to possible function of such polypeptides were given.
Non-amelogenin proteins have been identified in porcine immature enamel (Uchida et al., 1995). A 15 kDa protein had an N-terminal amino acid sequence Val-Pro-Ala-Phe-Pro-Arg-Gln-Pro-Gly-Thr-His-Gly-Val-Ala-Ser-Leu (SEQ ID NO:7) with no homology to previously known enamel proteins. It was proposed that the non-amelogenins comprise a new family of enamel proteins but their function was not suggested. The proteins have not been sequenced completety and their genes are not known.
WO89/08441 relates to a composition for use in inducing binding between parts of living mineralized tissue in which the active constituent originates from a precursor to dental enamel, so called enamel matrix. The composition induces binding by facilitating regeneration of mineralized tissue. The active constituent is part of a protein fraction and is characterized by having a molecular weight of up to about 40.000 kDa but no single protein is identified.